Chlorine and other halogens are frequently generated by an electrolysis of a brine of a salt of the halogen. Generally the salt is one of an alkali metal and the halogen. Cells used for this electrolytic process are subjected to a harsh chemical environment. Caustic products being produced in the cell, the brine, and the halogen being produced, together can cause a short service life for cell mechanical components. Particularly for electrodes such as the cell anode, where chlorine is evolved during electrolysis, service life can be troublesome.
While previously a relatively inert metal material for use in fabricating anodes for use in these cells was subject of diligent search, more recently electrodes fabricated from passivating or so-called valve metals have found wide acceptance in the generation of halogen by electrolysis. These valve metals commonly are considered to be titanium, tantalum, tungsten, bismuth, aluminum, niobium, zirconium and mixtures of these metals. Generally the valve metals tend to form a surface barrier layer when exposed to oxidants that tends to protect the valve metal from further damage. Often this barrier layer is not significantly electrically conductive.
One valve metal finding broad acceptance for fabricating electrodes for use in halogen generation cells is titanium. Titanium withstands corrosive effects of the cell environment well and is, at least relative to the other valve metals, suitably resistant to corrosive effects of contents of such halogen generating cells. Titanium offers relative availability and cost advantages as a metal for use in fabricating chlorine cell components. On an absolute scale, however, titanium remains relatively expensive, and where a large number of electrodes are required for use in a halogen generating plant, for example, the cost can be substantial.
Another drawback to the use of a pure titanium electrode is the electrical conductivity of the titanium, much less than copper, gold and silver, considerably less than baser metals such as iron and nickel, and substantially less than carbonacious materials such as graphite. When titanium is used to fabricate, particularly, a reticulate electrode widely used for halogen production, considerable care is required to ensure an adequate electrical current distribution throughout the reticulate structure. An inadequate current distribution can result in a relatively elevated power inefficiency in operating an electrolytic cell due to resistance losses as electrical current passes through the reticulate structure. Where the reticulate structure can be fabricated from a relatively conductive substrate having a coating of a valve metal for protection, considerable cost savings are available both in titanium costs and in potential costs of attachment of the reticulate electrode to a current feeder used to distribute electrical current.
Past proposals have attempted to provide titanium coated conductive substrates for use as an electrode by electrolytic deposition of titanium in an aqueous electrolyte. These coatings have generally been unsatisfactory where used in a halogen generating electrolysis cell. Contamination of the titanium coating, non-uniform thickness, relatively poor adhesion to the substrate, and small crystal size are among explanations offered for failures of these titanium coatings giving rise to the dissatisfaction.
In other proposals, pressure cladding of a conductive substrate with titanium has been tried for providing an effective coating. Cladding is the application of one metal to the surface of another using, generally, pressure to create an interdiffused zone between metal at the surface of the substrate and the coating metal. This interdiffused zone includes one or more alloys of the substrate metal and the cladding metal, the metals intertwining in a progression of crystal states corresponding to progressive changes in composition through the zone. This interdiffusion effect can strengthen bonding between the substrate and cladding metal promoting adherence.
In forming reticulate type electrodes, particularly pressure cladding can produce a less than satisfactory result. Particularly at corners or edges of a mesh screening used for reticulate type electrodes, this pressure cladding technique can produce less than a satisfactorily integrated coating. Where pores, or other irregularities are present in a clad coating, attack on the substrate by contents of an electrolytic cell in which the electrode is utilized can quickly cause spalling of coating around the cladding irregularity leading to rapid electrode failure.
For graphite substrates conventional cladding techniques can cause physical damage and achieve less than satisfactory adhesion, depending upon cladding conditions.
Techniques are known for the formation of interdiffused metal coatings upon a substrate using electrodeposition from a fused salt electrolysis bath such as "Flinak" an eutectic mixture of lithium, sodium and potassium fluoride having a melting point of about 454.degree. C. Such techniques are shown and described in U.S. Pat. No. 3,479,159, U.S. Pat. No. 3,864,221, French 1st Publication (Brevet) No. 2,075,857, and in 221 Scientific American 38 (1969). These references generally describe methods for forming solid solutions of coating metals on a substrate and intermetallics of the coating and a metal substrate, but do not describe desirably adequate techniques for providing a relatively pure coating of a valve metal on a relatively pure non-metallic conductive substrate such as graphite.
Upon a graphite substrate, efforts to deposit a valve metal from a fused salt electrolyte have met with a limited success in part due to electrolyte bath components such as potassium tending to become intercalated with the graphite substrate while the valve metal coating is being deposited upon the graphite producing a coating subject to spalling and other failures. Particularly where coatings have been applied under conditions conducive to substantial intercalation, those coatings often do not provide an acceptable base for the application of e.g. precious metal oxides to produce an electrode for use in a chloralkali cell or the like. Yet graphite, as a reasonably efficient electrical conductor, having an effective applied titanium coating could provide an effective electrode for use in a chloralkali cell. Aqueous electrodeposition techniques have generally been found unacceptable for making such a coating.